1. Field of the Invention
The present invention relates to a micro-switching device manufactured by a MEMS technique.
2. Description of the Related Art
In the technical field of wireless communication equipments such as a mobile phone, the increase components required to be incorporated in the equipment for achieving higher performance has been giving rise to a growing demand for RF circuits of smaller size. In order to meet this demand, a technique called micro-electromechanical systems (hereinafter, MEMS) has been employed for size reduction of various components constituting the circuit.
One of such components is a MEMS switch. The MEMS switch is a switching device that includes components fabricated in reduced sizes based on the MEMS technique, such as a pair of contacts that mechanically opens and closes for switching operation, and a driving mechanism that causes the pair of contacts to perform the mechanical switching operation, to name a few. The MEMS switch generally achieves higher isolation in an open state and lower insertion loss in a closed state than a switching device that includes a PIN diode or MESFET, especially when switching a high frequency signal of the order of GHz. This is because the open state is achieved by a mechanical opening motion between the contacts, and also because the mechanical switch incurs smaller parasitic capacitance. The MEMS switch is disclosed, for example, in patent documents such as JP-A-2004-1186, JP-A-2004-311394, JP-A-2005-293918, and JP-A-2005-528751.
FIGS. 25 to 29 depict a micro-switching device X4, as an example of the conventional micro-switching devices. FIG. 25 is a plan view of the micro-switching device X4, and FIG. 26 is a fragmentary plan view thereof. FIGS. 27 to 29 are cross-sectional views taken along the line XXVII-XXVII, XXVIII-XXVIII, and XXIX-XXIX in FIG. 25, respectively.
The micro-switching device X4 includes a base substrate S4, a fixing portion 41, a movable portion 42, a contact electrode 43, a pair of contact electrodes 44A, 44B (indicated by dash-dot lines in FIG. 26), a driving electrode 45, and a driving electrode 46 (indicated by dash-dot lines in FIG. 26).
The fixing portion 41 is joined to the base substrate S4 via a partition layer 47, as shown in FIGS. 27 to 29. The fixing portion 41 and the base substrate S4 are formed of monocrystalline silicon, and the partition layer 47 is formed of silicon dioxide.
The movable portion 42 includes, as shown in FIGS. 26 and 29, a stationary end 42a fixed to the fixing portion 41 and a free end 42b, and is disposed to extend along the base substrate S4 from the stationary end 42a, and surrounded by a slit 48. The movable portion 42 is formed of monocrystalline silicon.
The contact electrode 43 is located close to the free end 42b of the movable portion 42, as seen from FIG. 26. Each of the contact electrodes 44A, 44B is formed partially upright on the fixing portion 41 as shown in FIGS. 27 and 29, and includes a portion opposing the contact electrode 43. The contact electrodes 44A, 44B are connected to a predetermined circuit to be switched, via an interconnector (not shown). The contact electrodes 43, 44A, 44B are formed of an appropriate conductive material.
The driving electrode 45 is disposed to extend over a part of the movable portion 42 and of the fixing portion 41, as shown in FIG. 26. The driving electrode 46, as seen from FIG. 28, includes two upright posts jointed to the fixing portion 41 and a horizontal portion connected to the respective posts so as to span over the driving electrode 45. The driving electrode 46 is also grounded by a conductor (not shown). The driving electrodes 45, 46 are formed of an appropriate conductive material.
In the micro-switching device X4 thus constructed, when a potential is applied to the driving electrode 45, static attraction is generated between the driving electrodes 45, 46. When the applied potential is sufficiently high, the movable portion 42 extending along the base substrate S4 is elastically deformed until the contact electrode 43 makes contact with the contact electrodes 44A, 44B. That is how the micro-switching device X4 enters a closed state. Under the closed state, the contact electrode 43 serves as an electrical bridge between the pair of contact electrodes 44A, 44B, thereby allowing a current to run between the contact electrodes 44A, 44B. Thus, for example an on state of a high frequency signal can be attained.
On the other hand, in the micro-switching device X4 under the closed state, disconnecting the potential to the driving electrode 45, thereby canceling the static attraction acting between the driving electrodes 45, 46 causes the movable portion 42 to return to its natural state, so that the contact electrode 43 is separated from the contact electrodes 44A, 44B. That is how the micro-switching device X4 enters an open state as shown in FIGS. 27 and 29. Under the open state, the pair of contact electrodes 44A, 44B is electrically isolated and hence the current is inhibited from running between the contact electrodes 44A, 44B. Thus, for example an off state of the high frequency signal can be attained.
The micro-switching device X4 has the drawback that the contact electrode 43 suffers relatively large fluctuation in orientation toward the contact electrodes 44A, 44B.
In the manufacturing process of the micro-switching device X4, the contact electrode 43 is formed by a thin film formation technique on the movable portion 42, or on a position on the material substrate where the movable portion is to be formed. More specifically, a sputtering or a vapor deposition process is performed to deposit a predetermined conductive material on a predetermined surface, and the deposited layer is patterned so as to form the contact electrode 43. The contact electrode 43 thus formed via the thin film formation technique is prone to incur some internal stress. The internal stress often provokes deformation of the movable portion 42 at a position where the contact electrode 43 is adhered and the vicinity thereof, along with the contact electrode 43, as exaggeratedly illustrated in FIG. 30(a)-(b). Such deformation leads to relatively large difference (i.e. fluctuation) in orientation of the contact electrode 43 toward the contact electrodes 44A, 44B among each device.
The large fluctuation in orientation of the contact electrode 43 toward the-contact electrodes 44A, 44B leads to a higher potential to be applied to the driving electrode 45 in order to achieve the closed state of the micro-switching device X4. This is because it becomes necessary to set a sufficiently high driving voltage, to ensure that the device normally works irrespective of the extent of the orientation of the contact electrode 43 within an assumed range. Consequently, from the viewpoint of reduction of the driving voltage of the device, it is not desirable that the contact electrode 43 (movable contact electrode) has large fluctuation in orientation toward the contact electrodes 44A, 44B (stationary contact electrode).